Many commercial enclosed spaces need to be equipped with pressure relief ports which are sometimes referred to as equalization or ventilator ports. This is particularly true where the sealed space is subjected to temperature related gas volume variations that must be relieved.
Many of these enclosed spaces require that a positive air pressure differential to ambient be maintained. However there are spaces where no differential is required or desired. Passive ports are suitable for these. However existing passive pressure relief ports, meaning those without fans or blowers, have often permitted humid air migration where there is no significant pressure differential. With freezers this causes undesirable condensation and frosting. Frosting is a substantial problem that occurs as ambient warm air drawn into a low temperature chamber releases significant amounts of moisture relative to the change in dew point of the air at high and low temperatures. Air is drawn through the port after each door opening cycle as the warm air that entered cools and contracts. If venting does not occur, a partial vacuum results which make it difficult to reopen the door. In extreme cases, the enclosures can even collapse.
A temperature rise in the enclosure between cooling cycles, and especially during a defrost cycle, creates a need to vent air to prevent pressure buildup. Again, failure to vent this pressure, with adequate relief capacity, can cause the chamber to rupture.
Passive pressure relief ports are in wide commercial use today. However, they have a number of limitations and disadvantages. Many have in line valves located within housing that extend through walls. Their space requirements dictate that they be larger than the wall thickness and thus protrude from one or both sides of the wall. Where they extend into a freezer, it is difficult to prevent ice from forming, even with internal heaters. Their values are often spring loaded and thus over time lose sensitivity. This results in leaky valves that permit air and moisture migration and seepage, which can cause frosting and icing.
Accordingly, it is seen that a need exists for a passive pressure relief port, i.e. one that is not electrically powered by fans and baffles, which can relive both positive and negative pressure differentials, yet which substantially prevents humid air migration under static differential conditions.
Freezers have a propensity to build up ice or frost on the interior surfaces of the cabinet due to the exchange of ambient, humid air into the cold compartment. The ambient air, even in air conditioned environments, has a typical relative humidity of 50% to 70%. This humidity, or water content of the ambient air is quickly frozen when it enters the freezer and manifests itself as the formation of frost, snow, or ice on the interior and the contents of the freezer. This frozen condensation obscures product labels, interferes with product removal, and necessitates periodic defrosting. Modern domestic freezers have defrost cycles the melt away this frozen condensation at periodic intervals. In laboratory application of ultra low temperature freezers, a defrost cycle cannot be tolerated due to the potentially detrimental effects the heat of defrosting would have on the samples stored inside the freezer cabinet. Therefore, minimization or elimination of the frost in the interiors by means other than conventional warm surface defrosting is value to the laboratory user of ultra low temperature refrigeration.
Accordingly, it is desirable to provide a method and apparatus that overcomes the above disadvantages and problems.